This invention relates to a bandwidth changing circuit which may be used in FM receivers.
In the FM broadcasting, the bandwidth required for stereo broadcasting is different from that required for monaural broadcasting. In the former broadcasting, the required bandwidth is about 260 KHz, while in the latter broadcasting the required bandwidth is about 180 KHz. For this reason, the intermediate frequency bandwidth required for a stereo receiver is different from that required for a monaural receiver, but in any case the wider the intermediate frequency bandwidth, the higher the fidelity with which reception and reproduction are achieved. But, when, in case interference signals such as other broadcasting signals having a frequency approximate to a wanted signal frequency exists, the intermediate frequency bandwidth is wide, the interference signals sometimes enter into the intermediate frequency band to interrupt good quality of reception and reproduction due to the resulting beat interference.
For the above-mentioned reason, an FM receiver is provided with a bandwidth changing circuit for switching the intermediate frequency band from wideband to narrowband.
A conventional bandwidth changing circuit comprises a wideband signal path and a narrowband signal path including a narrowband filter which are arranged between a front end as a wideband signal source and an intermediate frequency amplifier stage, and a switching means for selectively coupling the wideband signal path or narrowband signal path between the front end and the intermediate frequency amplifier stage.
The amplitude level of an intermediate frequency signal when it passes through the wideband signal path is required to be substantially the same as that of the signal when it passes through the narrowband signal path. It is necessary, therefore, to provide on the narrowband signal path an amplifier for compensating the insertion loss of the narrowband filter, or alternatively to provide an attenuator on the wideband signal path. In the above-mentioned conventional bandwidth changing circuit, it is difficult to make its output impedance at selection of the wideband operation the same as the output impedance at selection of the narrowband operation.